Many implantable medical devices receive an externally generated data signal which may also act as the source of electrical power for the implant. Typically, data signals are transferred in such systems using Near Field Communication (NFC) in the high-frequency (HF) radio frequency (RF) band (3-30 MHz) over an electromagnetic field induction link. For example, a magnetic field induction (MFI) link can transmit and receive data between an external signal processor and the implanted device based on transformer-type induction between two aligned coils—one external and one internal.
The external signal processor in such applications can be thought of as a self powered initiator (e.g., by batteries), where the implanted device is a non-self powered target device that is remotely powered through the MFI link by extracting electrical energy from the transmitted RF data signal. The implanted device can answer to an external command to provide telemetry feedback data, for example, by load modulation of the transmitted signal by the implanted device. A telemetry circuit in the external signal processor then can demodulate this load-modulated RF feedback signal.
Digital data transmission generally occurs at a fixed data bit rate of some R bits/second. FIG. 1 shows the simple case of data bits as logic ONEs and ZEROs (possibly encoded) which are transmitted from the initiator device to the target device using on-off keying (OOK) modulation (which is a special case of amplitude shift-keying (ASK)). As seen in the bottom of FIG. 1, the RF carrier signal is a sinusoid wave with a fundamental frequency (fc) typically in the HF band. Data bit rates are typically less than or equal to fc/10 bits per second. Under low power constraints, a non-linear power amplifier (PA) such as a Class E amplifier modulates and amplifies the baseband signal at the initiator device producing the waveform shown at the bottom of FIG. 1. Demodulation and detection of the modulated OOK signal takes place at the target device to produce the signal shown at the top of FIG. 1.
Under low-complexity constraints, demodulation and detection make use of non-coherent schemes. That is, in contrast to coherent schemes based on phase-locked loops (PLLs) and Costas loops which are relatively complex to implement, in non-coherent approaches demodulation is performed without recovering the rf carrier and detection is performed without recovering the original timing. In the example shown in FIG. 1, the baseband signal is Manchester encoded so that a positive (negative) transition signifies a logic ONE (ZERO), and there is a signal transition at mid-bit. Note that independent of the bit stream and inherent to Manchester encodings, only two states are visible: either a double-wide HI (double-wide LO) or a single-wide HI (single-wide LO).
Low-complexity detection methods are commonly used which are based on asynchronous over-sampling and counting (O&C) algorithms, but these are not very robust against variations. In asynchronous over-sampling, the demodulated signal is sampled at some kR samples per second (k is usually a number greater than 3) by a clock unrelated to the encoder clock (no frequency or phase relationship between the clocks is imposed). The counting algorithm counts the samples in a HI (LO) state and decides based on a fixed decision interval (i.e. a discrete set of integers) whether the current count signifies a double-wide HI (double-wide LO) or a single-wide HI (single-wide LO). Decoding into a logic ONE/ZERO stream (i.e. a non-return to zero stream, NRZ stream) follows straightforwardly. Data detection is discussed at greater length in the following: U.S. Pat. Nos. 5,741,314; 6,600,955; 4,361,895; and U.S. Pat. No. 6,628,212; the contents of which are incorporated herein by reference.